In the magnetic recording of data for use in computer systems, various types of apparatus are utilized with each type presenting its own specific advantages. For instance, tape transports are used for the recording of large volumes of data accessible in a relatively short period of time, say a few seconds. The main advantage to the tape transport is that large volumes of data can be recorded on a single reel of relatively inexpensive tape and be somewhat readily accessible. The disadvantage, of course, is the time delay involved in having to wind the reel by the head until that data location on which the data desired is reached.
The need for immediate random access to recorded data has led to the development of various solid state memories such as core devices from which data can be read randomly and with only the access time necessary to switch electronic circuits. However, core devices are relatively expensive thereby cutting down on the number of installations in which such devices are used except mainly as buffer stores for computers.
Intermediate to the devices heretofore described there has been developed the disc recorder presenting the capability for random access to recorded data with an average access time depending upon the individual device, but in either event far less than that of the tape transport. With such devices a large volume of stored data is accessible on each disc with the data being recorded on tracks formed in concentric circles on the disc surface. Of course, if a more immediate access time is desired, drum devices having one head per track have been utilized, however, once again the total volume of data which can be recorded is normally less than that of a disc drive and the interchangeability of the recording medium is more difficult.
In the disc dirve there has been developed the use of disc packs in which several discs are stacked vertically with the drives having one head per disc surface and all the heads being coupled together to be accessed by a single linear acting motor. In such mechanisms a total cylinder of data can be accessed with the heads "on-line" that is located at the corresponding data tracks simultaneously, with a cylinder comprising the vertically stacked tracks of data (usually two per disc-one on the upper and one on the lower surface) at which the heads are positioned at any one time.
Since a large amount of the computer time is spent waiting on data to arrive for the various computations, the need for more on-line data transducers has naturally come about. However, there is a limit on the number of heads and discs which can be stacked vertically and accessed by a single mechanism since as the weight of the access mechanism increases with the number of heads, the access time for the driving motors utilized today naturally increases. The inertia of the head support mechanism is important since the mechanism is moved in a linear direction the same distance as the heads.
The primary answer to providing more on-line read/write heads for more immediate access to data at the present time is to provide a multiplicity of disc drives, usually up to eight in number, connected to a single controller through which data can be fed to the computer data channel. While the on-line data has been increased considerably by such a disc drive subsystem, the cost has also gone up because of the multiplicity of drives involved with a head access mechanism necessary for each pack. Therefore there still exists the need for drives having the capability of presenting more heads on-line preferably at a lower cost than the disc drive subsystems now utilized. Accordingly, the subject invention has as its primary object the provision of a less expensive disc drive apparatus having a quick access time and a greater number of on-line read/write heads, each capable of transmitting data between a disc magnetic surface and the computer system.